Project Summary Recognition of nucleic acids by the innate immune system is an ancient and fundamental mechanism of viral detection and host defense. Almost all of what we know about the mechanisms that link viral detection to activation of type I interferons and the antiviral response is based on the study of RNA recognition, particularly double-stranded RNA in the cytosol. In the last few years, the sensors and key signaling components that mediate dsRNA activated signaling have been defined. In contrast, it remained completely unknown whether an analogous system for cytosolic DMA detection existed or how such a system might function. We recently identified a unique innate immune response to cytosolic DMA and showed that it activates the antiviral response differently from RNA recognition. Based on these preliminary studies, the goal of this proposal is to define the unique signaling pathway activated by cytosolic DNA and examine its role in immunity. Elucidation of the principles of cytosolic DNA recognition will advance our understanding of innate immunity, antiviral responses, and self/non-self discrimination. Relevance An understanding of the mechanisms of cytosolic DNA recognition and the role of this system in immunity has fundamental implications for public health. Specifically, completion of the goals of this project will provide important insight into the basic mechanisms of the antiviral response, with implications for antiviral treatments, vaccine development, and autoimmunity. We will also perform experiments with direct relevance to a specific human disease, Aicardi-Goutieres Syndrome (AGS;OMIM 225750).